A magnetic disk drive includes read and write heads that may form a merged head attached to a slider on a positioning arm. During a read or write operation, the read and write heads are suspended over a magnetic disk on an air bearing surface (ABS). The read head has a sensor which is a critical component since it is used to detect magnetic field signals by a resistance change. One form of magnetoresistance is a spin valve magnetoresistance (SVMR) or giant magnetoresistance (GMR) which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. A higher resistance is observed when the free layer and pinned layer magnetization vectors are aligned parallel to one another than when they are aligned orthogonal to each other. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because when the power consumption in the sensor is made constant to avoid a temperature rise, the output voltage is roughly inversely proportional to the square root of the sensor area. Therefore, a stronger output signal is achieved as the sensor size decreases. The sensor area at the ABS plane for greater than 100 Gb/in2 density is smaller than 0.1×0.1 microns.
An important characteristic of a GMR head is the magnetoresistive (GMR) ratio which is dR/R where dR is the change in resistance of the spin valve sensor as the result of applying an external magnetic field and R is the resistance of the spin valve sensor before the change. A higher ratio is desired for improved sensitivity and it is well known that one way this result can be achieved is to incorporate a thin free layer in the spin valve structure. To further improve the GMR ratio, reduction of current shunting around the conductive layer (spacer) and structural growth optimization are needed.
Referring to FIG. 1, a conventional read head 1 based on a GMR configuration is shown and is comprised of a substrate 2 upon which a first shield 3 and a first gap layer 4 are formed. There is a GMR element comprised of a bottom portion 5, a spacer 6 such as a copper layer, and a top portion 7 that are sequentially formed on the first gap layer 4. The GMR element may be a bottom spin valve in which the bottom portion 5 is comprised of a lower seed layer, an AFM pinning layer, and an upper pinned layer (not shown) while the top portion 7 includes a free layer on the spacer 6. Alternatively, the GMR element may be a top spin valve where the pinned layer and AFM layer are sequentially formed above the spacer and the bottom portion 5 includes a free layer adjacent to the spacer. There are seed layers 8 and hard bias layers 9 disposed on the first gap layer 4 and along the GMR element. Leads 10 are formed on the hard bias layers 9 to carry current to and from the GMR element. The distance between the leads 10 defines the track width TW of the read head 1. Above the leads 10 and top portion 7 are formed a second gap layer 11 and a second shield 12. Although most of the sense current Is passes through the GMR element in this CPP configuration, a portion 13 of Is is shunted around the spacer 6 and reduces the output signal during a read operation.
For a bottom spin valve which is generally preferred over a top spin valve, decreasing the seed layer thickness and AFM layer thickness could help to reduce current shunting somewhat. A thinner seed layer and AFM layer could also provide an improvement in growth morphology (grain size and smoothness) that would increase the specularity of the seed layer/AFM layer interface and thus improve the GMR effect. For example, it is known that if an IrMn AFM layer is thinned from 70 to 55 Angstroms, there would be an immediate gain in dR/R of 7%. Since the interfaces between the various layers in a bottom spin valve stack are smoother as a result of a thinner AFM layer, the interlayer coupling between the AFM layer and free layer is desirably reduced as well. However, there is a limit to the minimum thickness for seed layers and AFM layers in current technology. Due to the finite size effect, thinning the AFM layer thickness will eventually cause the blocking temperature to drop significantly and thereby produce a potential hazard in thermal reliability. Furthermore, in a bottom spin valve, the seed layer must have large enough grains to promote the grain growth in the overlying AFM layer. If the seed layer is too thin, the AFM grains that contribute to the exchange bias cannot be established which results in a head stability problem.
Besides failing to achieve a large enough exchange bias field (HEX), improper AFM grain size will lead to a large magnetic training effect. A magnetic training effect refers to a reduction in the switching fields or HEX and HC as the magnetic field is repeatedly swept. For example, the magnitude of HEX and HC will shrink significantly after several cycles. Therefore, a method is needed that provides a thinner seed layer and AFM layer without reducing the blocking temperature (Tb) and compromising HEX.
Magnetic seed layers are also used in magnetic heads as described in U.S. Pat. No. 6,507,457 where a NiFeCr/FeCoZrTa seed layer configuration is used between an insulation layer and the top of a magnetic core. The FeCoZrTa top layer exhibits an increase in magnetic moment (Bs) after an annealing step.
In related art found in U.S. Pat. No. 6,326,637, a NiFe seed layer is inserted between a Pt seed layer and an OsMn AFM layer to improve the growth of the OsMn alloy. A high thermal stability and magnetoresistance is observed. However, the blocking temperature where the net magnetic moment no longer has a fixed orientation appears to be low even when Ir is added to OsMn to raise the Tb. Generally, a Tb of 250° C. or higher is needed so that the finished read head can withstand temperatures during subsequent processing.
In U.S. patent application 2004/0105193, a NiCr seed layer with 31 atomic % Cr is used to enable an overlying PtMn AFM layer in a bottom spin valve to be thinned to 80 Angstroms and thereby improves GMR performance.
A seed enhancement layer between a seed layer and a free layer in a top spin valve is disclosed in U.S. Pat. No. 6,496,337. Insertion of this seed enhancement layer that may be NiCu, for example, enables a thinner free layer and a higher GMR ratio.
In U.S. Pat. No. 6,222,707, a seed layer is comprised of NiFeCr or a NiFe/Cr multilayer and is selected to have a high resistivity to avoid shunting current away from the spin valve. Similarly, in U.S. Pat. No. 6,338,899 and U.S. Patent Application 2004/0121185, a material with high specific resistance such as NiFeCr is used in a Ru/NiFeCr composite seed layer in a bottom spin valve for shunt current control. U.S. Patent Application 2003/0143431 teaches the use of a low resistivity material such as NiFe alloy or Cr as a seed layer in a CPP type magnetic sensing element.